mix tapes (and the follow-on mix CDs that became popular with folk once CD-R media got cheap and CD-R drives got ubiquitous) were artifacts of a beautiful culture of citizen curators, rarely paid directly for their (usually statutorily prohibited!) curation, yet creating an entire market for (also illicit) distribution and marketing folk that definitely can be tracked to increased record sales for the artists included on the mixes.
I think about things like this pretty often and yet still not often enough.
capitalism ruins everything around me, and it has ever been thus.
on the flip side (ha), I've never once heard of a band that broke up because too much of their music was getting shared person-to-person for them to remain a going concern. I have, however, known and known of plenty of bands that gave up because they weren't getting heard.
now, I am quite sure there are folk in the current music industry who would point to the streaming services' playlist functionality and insist that being able to share playlists is the same thing but better because it's sanctioned (allowed, not sanctioned (forbidden)) but I disagree so hard.
part of the joy of a mix tape/cd was the fact that it was a physical artifact, a thing that someone made to put in your hand so you could enjoy it. there was an implied relationship between the recipient and the giver, and the chain of people between the original source and the giver could be super long or non-existent! it had a history, a back story, that artifact, that a playlist on a streaming service just doesn't, and a persistence that a playlist on a streaming service can never have.
Lady, EVOLVED 
@djsundog i would love to receive a mix CD, but even in the digital realm, the only playlist service which came close to replacing it was 8tracks
a significant amount of my music library is not on streaming services. if you won't let me upload my own music, you are useless to me.
(8tracks was great tho)
@Lady 8tracks and turntable.fm were both too beautiful to survive in their purest form for long yet indicated such a key lack in onlining the offline experiences involved
@djsundog (importantly, 8tracks was technically not a streaming service, but an internet radio service running highly personal tiny programs curated by human DJs)